FR2843244A1 - SPARK PLUG - Google Patents

Abstract

Spark plug with a ceramic component, the surface of which is at least partially coated with a glaze. This glaze contains between 0.6 and 4% by weight of fluoride and between 6 and 11.2% by weight of zinc oxide.

Description

Field of the Invention The invention relates to a spark plug whose

  ceramic components have a surface at least partly coated with a glaze. The invention also relates to a method of manufacturing such a spark plug. STATE OF THE ART Glazes are very widespread for protecting ceramic surfaces or quite simply ceramic surfaces. Such glazes are used inter alia also for insulators of

  i0 spark plug. The ceramic of the insulator is then protected by an appropriate glaze against the influence of the environment; its surface is smoothed and it is given a pleasant optical appearance. Whereas in the past, lead glazes were used for this, the use of lead in recent developments is avoided for reasons of pollution.

  Such lead-free glazes are for example known from document EP 788 204 AI; these glazes are made from borosilicate glass and they also have a low lead content. In addition, they contain fluoride in the form of sodium fluoride

or aluminum.

  The important criteria to have a good treatment of a glaze are among others its melting point and the stability of the bath obtained. Glazes having a sufficiently low melting point and a relatively high solidity are sought with respect to the crystallization and phase separation operations. Document EP 1 168 546 Ai describes glazes which have a zinc oxide content of 10 to 30 mol% to stabilize the glaze. The fluoride content is limited to a maximum of 1% mol; however the addition of fluoride is strictly

speak unwanted.

  OBJECT OF THE INVENTION The object of the present invention is to develop a glaze

  of lead-free spark plugs, with a low melting point and high stability in the molten state, which in the hardened state provides good bonding for the glaze surface and high mechanical strength and components of the candle glaze.

Statement of the invention

  To this end, the invention relates to a candle of the above type, characterized in that the glaze contains between 0.6 and 4% by weight of fluoride and between 6 and 11.2% by weight of zinc oxide.

  The candle according to the invention with the fluoride and zinc oxide contents in the glaze, has a low coefficient of thermal expansion and a low melting temperature below 900 ° C; this glaze offers particularly good stability in the molten state and advantageously forms in the hardened state, a smooth upper surface. The invention also relates to a method of manufacturing such a glaze. This process is characterized in that the glaze is made from a ceramic starting substance containing kaolin and / or bentonite to obtain a coefficient of thermal expansion

baked glaze <7 * 10-6 1 / K.

  The method according to the invention makes it possible to adapt the coefficients of thermal expansion of the glaze to those of the surface to be coated, in an advantageous manner so that after cooling and hardening, the glaze is in tension which makes it possible to obtain a high mechanical resistance of the components fitted with the glow of the spark plug. This appears in particular in the use of the glaze for spark plug insulators by giving them a bending resistance of

  the head, raised for the insulator with the glaze.

  The candle glaze may also contain barium and strontium. This greatly improves the insulation and water resistance characteristics of the glaze. In addition, there may be alkali oxides which further reduce the melting point of the glaze. It is particularly advantageous that the potassium oxide content is at least 1.5 times greater than the sodium oxide content.

  The glaze is particularly suitable as a surface coating for the spark plug insulator because it is very resistant to environmental influences and has good insulation properties.

Drawing

  The present invention will be described below in more detail with the aid of an exemplary embodiment of a spark plug shown in the single appended figure which is a sectional view of the spark plug.

  Description of the exemplary embodiment

  The spark plug 10 according to the present invention comprises a tubular metal body 13 housing a ceramic insulator 24. The insulator 24 surrounds at its end 27 located on the side of the combustion chamber a central electrode 22 and insulates it electrically with respect to the body 13. The body 13 also comprises a contact rod which transmits the voltage to the central electrode 22 and the connection end 28 of which comprises a connection nut 11. The connection nut 11 ensures the electrical contacting of the central electrode 22 at an external supply voltage not shown. This contact means mainly comprises a connection rod 12, the end of which located on the connection side includes a thread with a connection nut 19. Between the connection rod 11 and the contact rod 20, there is a resistor 25 formed of an electroconductive glass which ensures both the mechanical attachment of the components of the spark plug to the insulator 10 24 and a gas-tight closure with respect to the combustion pressure. Between the insulator 24 and the body 13 is an interior sealed seat 17 which seals the interior of the spark plug 10 relative to

the combustion chamber.

  The body 13 comprises one or more ground electrodes 15 21, welded. The ignition spark is formed between these ground electrodes

and the central electrode 22.

  The body or base 13 has on its outer side, a hexagon 14 for screwing the spark plug into the engine block. An external sealing seat 16 is also provided for sealingly separating the external atmosphere and the combustion chamber. The thread 18 printed in the base 13 is used to attach the spark plug to the engine block.

  The insulator 24 comprises at least on its outer side exposed to the ambient atmosphere, a vitrification 26 produced with glass

lead-free borosilicate.

  However, it is also possible to vitrify the insulator on other partial surfaces. The varnish has the following composition in weight percentages: SiO2 37.0 - 46.0 preferably 37.0 - 44.0 B203 12.0 - 28.0 preferably 17.5 - 23.0 A1203 4.0 - 21.0 preferably 8.5 16.0 ZnO 6.0- 11.4 preferably 7.8- 11.4 F- 0.6 - 4 preferably 0.6 3.0 Li20 1.5 - 4 preferably 1.9 - 3.5 Na2O 0.1 - 2.5 preferably 0. 1 - 2.0 K20 0.5 - 4.5 preferably 3.0 - 4.5 CaO 1.8 - 6 preferably 2. 1 - 4.2 SrO 0.1 - 3.6 preferably 0.1 - 1.2 BaO 0.8 - 6.8 preferably 4.5 - 6.5 The properties of glazes or enamels with the basic composition mentioned above were checked for the following glazes given by way of example. All figures are in percentages by weight. The quantitative indications of the various oxy5s relate to the glaze with the addition of corresponding amounts of kaolin or bentonite. For two basic glazes 1 and 5, a variable amount of kaolin was added to obtain glazes 2 to 4 and 6 to 9; the first glaze 1 has a higher content of zinccalciumstrontium oxide; the second base glaze 5 has on the contrary more oxide of

  sodium and potassium as the basic glaze 1.

  Trial 1 2 3 4 5 6 7 8 9 Kaolin O 10 20 30 O 10 15 20 30 Bentonite 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 SiO2 38.4 39.9 41.4 43.0 38.2 39.1 39.6 40.0 41.0

  B203 24.6 22.4 20.2 17.9 25.2 22.7 21.5 20.2 17.7

  A1203 5.1 8.5 12.1 15.7 5.1 8.5 10.2 11.9 15.3

  ZnO 12.1 11.0 9.9 8.8 11.3 10.2 9.6 9.1 7.9

  F 1.1 1. 0.9 0.8 0.9 0.8 0.8 0.7 0.6

  Li2O 2.7 2.4 2.2 1.9 2.9 2.6 2.5 2.3 2.0 Na2O 0.4 0.4 0.4 0.3 2.1 1.9 1.8 1.7 1.5

  K20 4.0 3.8 3.5 3.2 4.6 4.2 3.9 3.7 3.2

  CaO 4.0 3.7 3.3 2.9 3.0 2.7 2.6 2.4 2.1 SrO 1.1 1.0 0.9 0.8 0.1 0.1 0.1 0. 1 0.1 BaO 6.5 6.0 5.4 4.8 6.6 5.9 5.6 5.3 4.6

  WAK * 1OE 61) 7.1 6.7 6.1 5.6 7.6 - 6.7 -

  KBF2) 1725 1818 1804 1817 918 1209 1850 1834 1838 TE3) 454 446 444 437 - -

  1): Coefficient of thermal expansion, given in i / K; this coefficient was

measured between 200 and 4000C.

  2) Flexural strength of the head given as the average value in Newton measured according to the DIN ISO 11565 procedure. For the measurement, the spark plug to be checked was fixed with the greatest fixed tightening torque.

  in the corresponding standard for spark plugs in an appropriate test block. Perpendicular to the axis of the insulator and within a range of 5 mm at the end of the plug connection side, a force is applied and this force is increased until rupture.

  Amplitude of force is a measure of the flexural strength of the head.

  3) TE value as temperature in OC, for which the specific resistance of the glaze corresponds to 1 Mohmcm.

  Starting from the two basic glazes 1 and 5 it can be seen that the coefficient of thermal expansion decreases for a kaolin content greater than 10% by weight with at the same time an increase in the flexural strength of the head of the corresponding candle. Kaolin contents greater than 30% by weight do not bring a significant improvement in the properties of the corresponding glazes compared to those

  which contain 30% by weight in kaolin.

  To make the glaze we mix a glaze frit

  as a powder with kaolin or bentonite also in powder form; the content of kaolin and, if appropriate, of bentonite is chosen so that the fired glaze has a coefficient of thermal expansion <7 * 10-6 1 / K. Kaolin is essentially a clay containing kao10 lin; Kaolinite is a mineral aluminum hydrosilicate. Bentonite is a clay containing a mixed sodium aluminum hydroxide magnesium silicate.

  The raw materials in powder form are mixed with water or another solvent with the addition of an organic binder and they are applied by spraying, rolling or immersion on the insulator 24 to be vitrified. The thickness of the glaze layer is preferably between 5 and 40 μm. Finally, heat treatment is carried out at temperatures between 850 and 900 ° C; the insulator 24 is baked to obtain the glaze from the starting components.

Claims (1)

CLAIMS) Spark plug comprising a ceramic component, the surface of which is coated at least in part with a glaze, characterized in that s the glaze contains between 0.6 and 4% by weight of fluoride and between 6 and 11.2 % by weight of zinc oxide. ) Spark plug according to claim 1, characterized in that the glaze contains between 0.8 and 6.8% by weight of barium and / or 0.1 to 3.6% by weight of strontium. ) Spark plug according to claim 1, characterized in that the glaze contains between 37 and 46% by weight of silicon dioxide. ) Spark plug according to claim 1, characterized in that the glaze contains sodium and potassium oxide, and the potassium oxide content in weight percent is at least 1.5 times greater than that sodium oxide. ) Spark plug according to claim 1, characterized in that the glaze has between 0.5 and 4.5% by weight of potassium oxide.   6) Spark plug according to claim 1, characterized in that   the glaze does not contain iron oxide and zirconium.   ) Spark plug according to claim 1, characterized in that the temperature at which the glaze has a specific resistance of   1 MOhm * cm is between 400 and 520'C.   ) Spark plug insulator, characterized in that   it comprises a glaze according to at least any one of claims 1 to 7.   ) Method for manufacturing a spark plug, in particular according to one   any one of claims 1 to 7, according to which ceramic components of the spark plug are fitted with a glaze, characterized in that   the glaze is produced from a ceramic starting substance containing kaolin and / or bentonite to obtain a coefficient of thermal expansion of the baked glaze <7 * 10-6 1 / K.   ) Method according to claim 9, characterized in that up to 5% by weight of the starting ceramic substance is combined with   bentonite and / or 10-30% by weight of kaolin.   ) Method according to any one of claims 9 or 10, characterized in that   the ceramic starting substance is melted after addition of bento20 nite and / or kaolin at a temperature below 900 ° C.